Sepsis is the leading cause of death among critically ill patients in the United States with between 230,000 and 370,000 people dying from the disease annually. Outside of antibiotics, treatment for sepsis is non- specific, and there is no approved therapeutics available once antibiotics and supportive therapy fail. The gut has long been characterized as the motor of multiple organ dysfunction syndrome. We have spent the two previous cycles of funding examining mechanisms of gut apoptosis in sepsis and have come to believe that understanding the gut in sepsis requires a broader view of the interconnectivity of the organ. This proposal therefore focuses on understanding how components of the gut epithelium interact in driving sepsis, as well as understanding how the gut epithelium and the adaptive immune system alter each other following sepsis. The first goal of the proposal is to understand why sepsis slows migration along the crypt-villus axis. By using mice with gut-specific alterations in apoptosis or proliferation, we can test whether intestinal migration is dependent upon apoptosis or proliferation. Further, by inducing proliferation in the villus (a compartment in which gut cells do not ordinarily proliferate), we can delineate the importance of intestinal microenvironment in modulating sepsis-induced decreases in migration. Gut integrity must also be understood in the context of how the intestinal epithelium functions as a barrier between the intestinal microbiome and the external environment. As such, the proposal seeks to understand mechanisms underlying sepsis-induced hyperpermeability and how the adaptive immune system modulates changes in the intestinal barrier following sepsis. Studies will be performed using epidermal growth factor (EGF), a cytoprotective growth hormone, which improves survival and normalizes gut barrier dysfunction when given to immunocompetent septic mice but paradoxically worsens survival without barrier improvement when given to mice lacking lymphocytes. Comparing the effects of EGF and other barrier-altering agents when given to immunocompetent or lymphocyte-deficient mice will provide insights into how the gut barrier impacts survival and how this is altered by T cells following sepsis. Since the gut plays a major role in both initiating and propagating critical illness, understanding mechanisms through which gut integrity is dysregulated in sepsis has significant public health implications in a disease that is common, very costly, and highly lethal.